The present invention relates generally to a method for recovering ethane from a hydrocarbon gas stream. More particularly, the present invention relates to an improved method having turboexpansion of a methane-rich stream to provide refrigeration in an ethane extraction process which is used to recover an ethane product.
The production and consumption of ethane is rising because it is a valuable petrochemical feedstock for ethylene manufacture. Furthermore, highly pure methane and propane are also valuable products of an ethane extraction plant.
FIG. 1 is simplified schematic of a typical ethane extraction plant. A hydrocarbon feed, containing methane, ethane, propane and possibly heavier hydrocarbons, is fed to a demethanizer column. The overhead of the demethanizer column is typically a methane-rich stream. The bottom of the demethanizer column is fed to a de-ethanizer column. An ethane-rich stream is recovered overhead and a propane-rich stream is recovered from the bottom of the de-ethanizer column.
The hydrocarbon feed is often cooled through a refrigeration section (not shown) prior to entering the demethanizer column. Refrigeration is a useful technique for achieving low temperatures necessary for the separation of methane from the other hydrocarbon constituents in the hydrocarbon feed.
Several methods currently exist for recovering methane, ethane and propane from hydrocarbon gas streams. Some typical examples of isolating and extracting ethane and or propane are disclosed in U.S. Pat. Nos. 4,278,457; 4,597,788; 4,689,063; 4,851,020; 4,869,740; 5,275,005; 5,568,737; 5,588,306; 5,890,377; and 5,953,935.
Existing ethane extraction plants use (i) propane refrigerant as the cooling means for ethane separation or (ii) a combination of turboexpansion and Joule-Thompson expansion of the hydrocarbon feed. The use of turboexpansion on a gas feed to an ethane extraction plant is possible when the gas stream is lean, i.e., having a low concentration of hydrocarbons heavier than methane. When a gas stream is rich in ethane or propane, these hydrocarbons would typically condense into liquids during cooling, resulting in insufficient cooling at the turboexpansion facilities associated with the gas feed.
For a gas stream rich in ethane or propane, however, a propane refrigeration system is commonly used to cool the gas feed stream. Propane refrigeration, however, often becomes inefficient with lean feeds.
A cold methane stream has been used to partially cool the gas feed to an ethane extraction plant. A process disclosed in U.S. Pat. No. 4,687,499 used a cool overhead methane stream from a demethanizer column as a cooling source for a natural gas feed. The cool demethanizer overhead methane stream cooled the natural gas feed in a heat exchanger. The methane stream was then compressed before being recovered as a methane product. This process, however, is limited in overall efficiency because it does not fully utilize the cooling potential of the methane stream.
There is a need for an ethane extraction process capable of efficiently and economically extracting ethane from both lean and rich gas feeds.
The present invention is a process for recovering ethane from a hydrocarbon gas feed stream containing methane, ethane, propane and possibly heavier hydrocarbons. Cooling of the feed stream is accomplished in an exchanger, such as a cryogenic heat exchanger. Turboexpansion of a methane-rich stream provides a cooling source for the cryogenic heat exchanger to cool the hydrocarbon gas feed.
The cooled hydrocarbon gas feed is separated into a methane-rich stream and an ethane/propane-rich stream in a demethanizer column. An ethane-rich stream is recovered from a de-ethanizer column.
The turboexpansion of a methane-rich streams permits the processing of lean and/or rich hydrocarbon feed streams while avoiding undesirable condensation that may lower cooling capability of the refrigeration system, such as the cryogenic heat exchanger.
In one aspect, the present invention is a method for recovering ethane from a hydrocarbon gas stream having methane, ethane and propane which includes the steps of (i) providing the hydrocarbon gas stream; (ii) cooling the hydrocarbon gas stream by refrigeration to form a cooled hydrocarbon gas stream; (iii) separating the cooled hydrocarbon gas stream into a methane-rich stream and an ethane/propane-rich stream; (iv) expanding the methane-rich stream from a high pressure to a lower pressure to lower the temperature of the methane-rich stream which then provides a cooling source for the refrigeration of the hydrocarbon gas stream; (v) separating the ethane/propane-rich stream into an ethane-rich stream and a propane-rich stream; and (vi) recovering methane-rich, ethane-rich and propane-rich product streams.
Desirably the step of expanding the methane-rich stream includes the steps of (i) compressing the methane-rich stream; (ii) cooling the compressed methane-rich stream; and (iii) turboexpanding this cooled and compressed methane-rich stream.